The RegExp() constructor directly returns the pattern argument only if pattern is a regex (among a few other conditions).() and replaceAll() check whether the global flag is set if the first argument is a regex before invoking its or method.(), startsWith(), and includes() throw a TypeError if the first argument is a regex.( exec could also be used, but because it's not a symbol property, there would be too many false positives.) The places that treat regexes specially include: This choice was made because is the most indicative property that something is intended to be used for matching. A non- RegExp object with a Symbol.match property will be treated as if it's a regex.An actual RegExp object whose Symbol.match property's value is falsy but not undefined (even with everything else intact, like exec and can be used as if it's not a regex.Note that in most cases, it would go through the Symbol.match check, which means: (This step should rarely happen, since if x is a RegExp object that have not been tampered with, it should have a Symbol.match property.) Otherwise, if x is undefined, check if x had been created with the RegExp constructor.If x is not undefined, check if it's truthy.They decide whether x is a regex through multiple steps: Some built-in methods would treat regexes specially. Note: Whether something is a 'regex' can be duck-typed. Object.prototype._lookupSetter_() Deprecated.Object.prototype._lookupGetter_() Deprecated.Object.prototype._defineSetter_() Deprecated.Object.prototype._defineGetter_() Deprecated.

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